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Wood.                                     87

(14.) WoQ,d is not used to so great an extent as formerly.
Roofs are made of wrought iron; and men-of-war of iron and
steel instead of oak: pillars of cast iron : while morticed wheel
teeth are almost out of fashion. Brake blocks, too, are made of
cast iron, to give a longer time of wear -3 and wooden buffer beams
for locomotives are now being discarded.

Little then need be said of wood. For pattern-making, as
already stated, jbtne, mahogany, cherry, sycamore, lime tree, and
walnut are the woods used. JLnglish oak is the best for beams,
but American oak is much cheaper, and the latter is used for the
framing of railway and traction waggons, and for locomotive
buffer beams. Ash is also much employed in waggon work,
especially for cart shafts. Mortice teeth are made of beedi or
hornbeam* Lignum vitas is of great service for bearings that are ,
immersed in water as, for example, mth the screw-propeller and
some turbines. (See Appendix 77~.,/. 787.)

Railway sleepers are rendered very durable by impregnation
with creosote or black oil, air being first sucked from the pores of
the wood. The creosote is then forced in at great pressure.

The following table gives the melting points in degrees
Fahrenheit of the principal metals mentioned in this chapter :—

Cast Iron   __.....    2ioo°P.

Wrought Iron____    3000° *

Steel   ...____.....    2700°


Zinc   ..............      773°F-

Tin.................      44V

Gun metal......—    1900".

Brass......   1700° to 1900°

v   Soft Soldering.—See Appendix K, ^.970,

* Castings of * -wrought iron' have been-made^' though   the process .is-
somewhat   intricate,   and has not ,been extensively , applied.    The method .
consists principally in lowering the high melting point of wrought iron by the
addition of aluminium.    Swedish wrought iron is used,' and from T
of its weight of aluminium is mixed with it,                               "